A shipment of 36 million pounds of soybeans sailed late last year from Ukraine to Turkey to California. Along the way, it underwent a remarkable transformation. The cargo began as ordinary soybeans. They were fumigated with a pesticide [and] priced like ordinary soybeans. But by the time the 600-foot cargo ship carrying them to Stockton, Calif., arrived in December, the soybeans had been labeled organic," according to receipts, invoices and other shipping records. That switch - the addition of the "USDA Organic" designation - boosted their value by approximately $4 million, creating a windfall for at least one company in the supply chain. About 21 million pounds of the soybeans have already been distributed to customers. The multimillion-dollar metamorphosis of the soybeans, as well as two other similar grain shipments in the past year examined by The Post, demonstrate weaknesses in the way that the United States ensures that what is sold as "USDA Organic" is really organic. The three shipments, each involving millions of pounds of "organic" corn or soybeans, were large enough to constitute a meaningful proportion of the U.S. supply of those commodities. All three were presented as organic, despite evidence to the contrary. USDA officials say that their system for guarding against fraud is robust. The system suffers from multiple weaknesses: Farmers hire their own inspection companies; most inspections ... lack the element of surprise; and testing for pesticides is the exception rather than the rule.